The present invention relates to the detection of faults in electrical circuits and, more particularly, to the detection of faults in circuits which have both AC and DC sections.
Detection of faults in electrical and/or electronic circuits is becoming increasingly important in many applications, especially in those applications where faults can have severe results. For example, failure to detect a fault in the electrical power generation equipment on board aircraft may result in more complex and expensive power generation systems and damage to the electrical utilization loads on board the aircraft. It is advantageous, therefore, to detect faults whether they occur in the electrical power generation apparatus itself or in other elements of the electrical system.
The electrical power generation system of an aircraft typically includes an alternator or generator driven by a prime mover such as an engine of an aircraft. The output of the generator or alternator is connected through a rectifying bridge in order to convert the AC supplied by the generator or alternator into DC. This DC can be used directly by the DC loads of the aircraft and can also be inverted into constant frequency AC for supply to the aircraft's AC loads. One of the most common faults in electrical power generation equipment of this type is a short of one of the diodes in the AC to DC converter. A shorted diode in the AC to DC converter can introduce AC onto the DC load bus resulting in damage to the DC loads. A shorted diode can also result in high phase currents from the generator or alternator which can potentially cause further damage to the AC to DC converter, low line-to-line AC voltage, and a high DC ripple voltage to the DC to AC inverter which can cause the inverter to produce harmonics requiring heavy and expensive harmonic filters. These consequences make it necessary to detect and disconnect a rectifier bridge with a shorted diode in as short a time as possible. Preferably, the sensing circuit should react in less time than the switching time of the switch which disconnects the converter from the DC bus in response to the sensing circuit.
Measuring the output of the converter alone may be insufficient in order to detect a fault in the converter. For example, a shorted diode in the converter may result either in relatively little change in the magnitude of the DC output from the converter or in a decrease in DC current which could have resulted from changing load conditions instead of a fault in the AC to DC converter. Therefore, it is necessary to sense parameters involved in the conversion of AC into DC which will provide a useful indication of a fault in a converter.